Hydraulic pressure is commonly used to power equipment such as generators, compressors, power tools, etc. Frequently, a power take off is used with a truck to provide power to hydraulic equipment. The power take off typically connects a hydraulic pump to a truck transmission. The hydraulic pump is connected to inflow and outflow lines that terminate in hydraulic quick release connectors at the back of the truck.
Problems during use of the power take off and hydraulic equipment can damage the equipment. As an example, if the return line from the equipment to the pump is not properly connected, the return line remains closed and the hydraulic equipment experiences the full output pressure of the hydraulic pump as soon as the pump is engaged. Hydraulic equipment typically contains pressure sensitive components such as hydraulic fluid coolers on the return side of the hydraulic circuit. Subjecting these components to full hydraulic pressure will cause them to burst and will spill the hydraulic fluid as quickly as the truck hydraulic pump can pump the fluid.
It is easily appreciated that it is desirable to protect the hydraulic equipment from overpressure situations in order to eliminate damage and to prevent hydraulic fluid spills.